Throughout the ages past, war has been used in many different ways. Blood feuds, conquest, territorial disputes, religion, terrorism, policing actions, and politics. But, no matter how you swing it, war, for whatever purpose or motive, is not a good thing.
Certainly one can argue that, if Hitler had not been stopped in the 1940's, he would have taken over the world. And so, we go to defend our country, our way of life, and the world from evil. That doesn't take away the brutality and corruption that can result from conflicts as large as these.
There are examples in our past of wars that were negative in nature. The Vietnam War, a fine example of political tensions causing the deaths of thousands of people. The Cuban Missile Crisis, where the hatred between two superpowers nearly caused a nuclear holocaust. The Crusades, where religion was used as an excuse to gain an influential piece of territory in Israel.
War is a tool. Just like the gun, the law, technology, and the vote, war can be used either for good or for evil. Whether it be rebellion against an oppressor as we saw in World War II, or the same scenario where one man desired power above all else.
Regardless of it's use, war brings devastation, deprivation, and death.